Polycarbonate can be prepared for shipment by manufacturers in forms which differ to a significant degree in terms of size. For example, polycarbonate pellets which are too large to pass through a sieve having a mesh opening size in the range of about 2.5 to 4 mm are known. However, polycarbonate in powder form is also known where the granules may range in size from less than 100 to as much as 2,000 microns.
Polycarbonate in powdered form is typically prepared by chopping or crushing polycarbonate which is not melted or dissolved. For example, in Narita, U.S. Pat. No. 4,074,864, methods and apparatus are disclosed for continuous production of polycarbonate powder from a polycarbonate solution. This involves mixing and kneading a polycarbonate solution while heating same and while simultaneously repeating the feeding of the solution forward and backward with the aid of meshing spiral blades in a twin screw extruder. As the solvent in the polycarbonate solution is evaporated by the applied heat, the product undergoes drying, grinding and powdering and is then discharged through an cutlet in powder form. Although the polycarbonate powder is reported in Narita to have an average grain size of 1,700 microns, a minor portion of it is as small as 200 mesh or less. Koda, U.S. Pat. No. 4,184,911, also discloses a process for producing powdery polycarbonate from a polycarbonate solution by (a) charging the solution into a desolvating apparatus which is constituted of at least two intermeshing screws in a casing, the casing having an evaporating zone and a powdering zone: (b) evaporating the solvent in the evaporating zone by crushing between the screws: (c) powdering the dried polycarbonate in the powdering zone: and (d) discharging the powdery polycarbonate from the product exit in the powdering zone.
Handling any substance in powder form presents a problem with dust (extremely fine particles which become airborne) and the consequent loss of material and increased difficulty of housekeeping. Material in powder form is also more difficult to clean from an extruder when there is a change of feedstock and is frequently more difficult to melt uniformly in an extruder. Although polycarbonate in pellet form is generally not subject to such difficulties as are inherent in powder, even the usefulness of polycarbonate pellets often depends on the appropriateness of the pellet size for the intended operation. For example, pellets which have too broad a size distribution are more difficult to convey in a pneumatic system than pellets which are 85% or more within a specified size range.
Accordingly, it would be desirable to have an easily practiced method of producing polycarbonate in the form of pellets characterized by specified size range and distribution so that the convenience of being able to handle polycarbonate in pellet form would be available regardless of the size of pellet needed.